1. Field of the Invention
This invention relates to a method for regulating at least one parameter in a reactor used to chemically produce end product from two or more substances. The invention further relates to means for controlling the temperature at which a chlorinating agent and reducing agent react with metal compound to form metal chloride in a reactor. The invention represents an improved method for producing magnesium chloride from magnesium oxide-containing compounds, including magnesite.
2. Technology Review
In U.S. Pat. No. 4,269,816, the disclosure of which is incorporated herein by reference, there is claimed a process for preparing magnesium chloride from magnesium carbonate, carbon monoxide and gaseous chlorine. The process commences by heating pieces of magnesium carbonate in a packed bed reactor to a temperature above the melting point of magnesium chloride and below about 1200.degree. C. The heated magnesium carbonate pieces of this packed bed are then reacted with gaseous chlorine in the presence of carbon monoxide. Carbon dioxide is then withdrawn from above the bed while molten anhydrous magnesium chloride is withdrawn from below said packed bed. The temperature of reaction within the foregoing reactor is typically maintained between about 800-1000.degree. C. At such high temperatures, some of the magnesium chloride that forms vaporizes. When offgases are removed from this reactor, vaporized end product (magnesium chloride) escapes thus lowering process efficiency.
When a metal oxide is converted to metal chloride, the heat produced from this exothermic reaction provides much of the energy needed to maintain the reactor at a desired temperature. If too much heat is present, active cooling means must be employed to prevent excessive losses due to end product vaporization. Various mechanisms have been employed for externally cooling reactor vessels of this sort. The most straightforward external cooling means consists of immersing the reactor or constantly pouring liquid coolant, such as water, over the same. This type of cooling means does not provide means for proceeding at various temperatures, however. The extent of cooling is also dependent upon the type of liquid coolant used, overall reactor size and shape, and the temperature of reaction within said reactor.
Modifications in reactor vessel size are another alternative means for controlling temperature within a reaction vessel. Increasing the total height of a given reactor area may improve thermal conductivity but at increased construction and/or operation costs. Decreases in vessel liner thicknesses are also possible. Thinner liners would tend to place greater temperature strains on the external shell of said reactor vessel. Finally, temperature control means may also be achieved by purposefully recycling at least some portion of cooled end product back into the system. The latter approach reduces process efficiency by sacrificing already formed end product for greater reaction temperature control.
The present invention has determined that the temperature of reaction in an exothermic chlorination reactor can be controlled by pre-reacting at least some portion of the feed gases outside of the main reaction zone. When chlorine is the preferred chlorinating agent and carbon monoxide the preferred reducing agent, these two gases may be combined externally to form phosgene. The heat of reaction associated with phosgene formation is then absorbed in its own continuously-cooled vessel before phosgene is directly introduced into the main reaction zone. When the amounts of chlorine and carbon monoxide diverted from the main reactor for preconversion are purposefully varied, the overall temperature of reaction within said vessel may be more variably adjusted or controlled. This invention also significantly lowers offgas temperature within the main reactor.
In British Patent Specification No. 718,773, a method for converting aluminum oxide to aluminum chloride was proposed which included mixing together equal volumes of carbon monoxide and chlorine. The resulting phosgene was then fed, without cooling, into the main reactor vessel. In this manner, the aluminum chloride production method of British Patent Specification No. 718,773 made use of hot phosgene (and the heat set free by combining carbon monoxide and chlorine) to enhance reaction efficiency. In more preferred embodiments, hot phosgene was fed into the main chamber of this aluminum chloride reactor at about 500.degree. C. for promoting a total reaction temperature between about 500-800.degree. C. therein.